Not a problem as such but you will have to map DSCP to CoS to go over the WAN and them map CoS back to DSCP at the other end. This is assuming that you want to prioritise the traffic on the WAN. If you don't then simply leave the DSCP value as is.
Neither DSCP or CoS (802.1p) prioritize automatically. It s up to you to define what level of service you want to allocate to specific markings. Note that you have said L3 QOS is dscp and CoS but it isn't. L3 is DSCP and ToS (Type of Service). CoS is L2.
So DSCP and CoS merely mark the packet. It is then up to you to define how you want that value to be treated ie. priority queued, a certain level of bandwidth allocated, shaped or policed to a certain amount.
DSCP allows for 64 values. CoS and ToS are much more limited, 8 values only, so when you map back and forth between them you can lose some granularity.
But you can define the same treatment for DSCP EF and CoS map and then map VOIP into both.
Could there be problem on the WAN link if it only supports 802.1p? It depends.
It's not so much a question of L2 or L3 markings, but whether you have any congestion that adversely impacts your traffic and whether you can manage it.
Markings, whether L2 or L3, are just a handy tag for downstream devices to quickly recognize a set of traffic that should be treated a certain way.
What's generally important for VoIP, is to insure such traffic has the bandwidth it needs and it not impeded by other traffic sharing a path.
Likely, you'll want to insure your VoIP is dequeued first, including when provided to the WAN, and if the WAN is some kind of "cloud", insure the VoIP is marked so the cloud provider can also insure VoIP goes first if there is congestion.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...